The oil activity surface (OAS), which is a 3D plot of the effective oil adsorption energy as a function of the estimated contact temperature and load, is used as a methodology to study the performance of two ZDDP antiwear additives. The paper presents a boundary lubrication model which permits evaluation of the adsorption energy of a binary lubricant system. Binary oil system performance was studied by determining the adsorption energy with respect to that of the base oil alone and measuring the wear rate inside the experimental operation condition space defined for the entire test set. The adsorption energy depends on the composition of the film formed from ZDDPs, oil temperature, applied load and basestock nature. This is a measure of the degradation of ZDDPs and can be associated, depending on base oil, with the antiwear performance of ZDDPs.